The present invention relates to the generation of particulate beams characterized by high brightness and small emission area, and more particularly, to an apparatus and method for the generation of neutral and anionic particulate beams. Even more particularly, the present invention generates anionic and neutral fullerene beams. The present invention also relates to a method for generating neutral and anionic particulate beams, and more particularly to a method for generating anionic and neutral fullerene beams. The present invention also relates to a system that utilizes a particulate beam for analyzing substances ejected from a surface of a sample bombarded with the particulate beam.
Fullerenes:
Fullerenes, most notably C60, are a newly discovered form of carbon. The fullerenes are a family of hollow (cage) all-carbon structures. C60 is the most prominent member of this family. C60 is a perfectly symmetrical molecule composed of 60 carbon atoms arranged on the surface of a sphere in an array of 12 pentagons and 20 hexagons (a soccer-ball molecule). C60 has many unique properties but most relevant here arc its structural rigidity (closed cage) and its thermal and collisional stability.
Other relatively common fullerenes are C70, C76 and C84. Their structure is described in [“Science of fullerenes and carbon Nanotubes,” M. S. Dresselhaus et. al., Academic Press, San-Diego 1996] which is incorporated herein by reference. Fullerene cages are approximately 7–15 Angstroms in diameter. The molecules are relatively stable; the molecules dissociate at temperatures above 1000 C. Fullerenes sublime at much lower temperatures, i.e., a few hundred degrees C.
Neutral and Anionic Particulate Beams
The production of neutral and anionic particulate beams is of considerable importance in such diverse areas as atomic, molecular and plasma physics, thin film deposition, surface etching, ion implantation, submicron lithography, nano-electro-mechanical and nanophotonic system construction, new material synthesis, and electric propulsion devices. Applications utilizing anionic particulate beams find use in fundamental science areas, e.g., surface chemistry and catalysis, organic chemistry, and biology. For example, FAB (Fast Atom Bombardment) and TOF-SIMS (Time Of Flight Secondary Ion Mass Spectrometry) instruments are widely used for tailoring and analyzing new biomaterials and organic structures on the molecular level in the fields of pharmacology and biotechnology.
The use of energetic cluster or polyatomic neutrals or ions as primary projectiles for static SIMS analysis of organic and inorganic samples has many advantages compared to the traditionally used atomic ion collider. Polyatomic or cluster ions produce significantly higher yield of secondary ions (5–100 times) as compared to atomic ions. This yield enhancement relates to the fact that the deposited impact energy is distributed over a broader surface region than for an atomic species. Therefore, the use of fullerene ion projectiles as the primary beam is attractive due to the shallow penetration of the fullerene ion projectile into the bulk and the extremely high surface sensitivity of the adsorbed molecules analysis.
The most important features of ion sources used for SIMS applications and for submicron-level micro fabrication are maximal brightness and minimal emission area of the beam. These two parameters enable both tight focusing of the beam for surface imaging (nanoprobe beam formation) and a high beam density for dynamic SIMS depth profiling. Various methods for the generation of positive and negative fullerene ion beams have been used, e.g., laser ablation and desorption of graphite or fullerene targets [M S Dresselhaus et al., “Science of Fullerenes and Carbon Nanotubes”, Academic Press, San Diego, Calif., 1996; HD Busmann et al., “Surface Science”, 272: 146, 1992], fission fragments impact on a C60 coated surfaces [K Baudin et al., “A Spontaneous Desorption Source For Polyatomic Ion Production”, Rapid Comm. in Mass Spect. 12 (13): 852–856, 1998], fullerene thermal desorption combined with electron attachment or electron impact ionization [T Jaffke et al., “Formation of C60− and C70− By Free Electron Capture. Activation Energy And Effect of the Internal Energy On Lifetime”, Chem. Phys. Lett. 226: 213–218, 1994; SCC Wong et al., “Development Of A C-60(├) Ion Gun for Static SIMS and Chemical Imaging”, Appl. Surf. Sci. 203: 219–222, 2003; D Weibel et al., “A C-60 Primary Ion Beam System For Time of Flight Secondary Ion Mass Spectrometry: Its Development and Secondary Ion Yield Characteristics”, Anal. Chem. 75 (7): 1754–1764, 2003]. Attempts have also been made to use conventional ion sources (arc-discharge and sputtering type) [PD Horak et al., “Broad Fullerene-Ion Beam Generation and Bombardment Effects”, Applied Physics Letters, 65 (8): 968–970, 1994; S Biri et al., “Production of Multiply Charged Fullerene and Carbon Cluster Beams by a 14.5 GHz ECR Ion Source”, Review of Sci. Instr. 73(2): 881–883, 2002; C Sun et al., “Extraction of C60− and Carbon Cluster Ion Beams from a Cs Sputtering Negative Ion Source”, Fudan Univ., Shanghai, Peop. Rep. China. Hejishu 17(7): 407–410, 1994]. These methods have various drawbacks when used for submicron focused beam applications. Among these are the complexity of the source, the need for an additional mass filter due to fragmentation upon ionization, low current density and brightness, and large energy dispersion of ions or poor focusing.
It is well known that for many polyatomic molecules the attachment cross section at near zero electron kinetic energy can be quite large. For example, direct interaction of fullerenes with thermal electrons produces very long-lived metastable anions. The energy due to the captured extra electron (comprised of the kinetic energy of the free electron plus the molecular electron affinity) is effectively dissipated among the vibrations of the molecular ion. The ion may decay via delayed (10 μs–10 ms) autodetachment.
A typical prior art apparatus for the generation of molecular anions includes a monochromatic electron source for providing the low energy electron beam (0.1–2 eV) [E Illenberg et al., “Gaseous molecular ions. An Introduction to Elementary Processes Induced By Ionization” (Stenkopff/Springer, Darmstadt, Berlin), 1992]. The electron beam is crossed at a right angle to a molecular beam effusing from a capillary. The capillary is connected to an oven containing a fullerene sample. The oven is kept at the temperature in the range of 600–800 K. Negative ions formed by electron capture are extracted from the reaction volume by a weak electric field and are accelerated to a given energy onto the entrance of the ion beam formation system. The main disadvantage of this method is low beam brightness due to the large ionization volume needed to generate high ion current and an inability to introduce a strong electrostatic field into the reaction volume as needed due to strong effects of external fields on trajectories and energy of electrons and depression of the ionization process.
Reference is now made to FIG. 1, which is a schematic illustration of a prior art apparatus 20 for the generation of fullerene negative ions based on a surface ionization process. In a surface ionization process, a plurality of neutral molecules is adsorbed onto a hot surface with a low work function. A portion of the plurality of neutral molecules is then ionized as the molecules emitted from the surface. The prior art apparatus is described in Russian Patent No. 2074451 to L. N. Sidorov, et al.
Apparatus 20 of FIG. 1 comprises an internal effusive cell 22 nested inside an external effusive cell 24. Internal cell 22 has an effusive orifice 30 and contains a fullerene mixture powder 26. External cell 24 also has an effusive orifice 32 and contains a material 28 that reduces the work function of its walls. In the reported method, material 28 is a mixture of AlF3+KF. Cells 22 and 24 are manufactured from nickel.
Cell 22 and cell 24 are heated simultaneously so that the internal pressure of the nested cells 22 and 24 reaches the equilibrium vapor pressure of fullerene. Negative surface ionization of the plurality of fullerenes takes place on the walls of external cell 24. The ionized molecules are extracted from orifice 32 on the front conical part of external cell 24. The ionized molecules are accelerated by the applied electric field (not shown).
The apparatus of FIG. 1 is disadvantageous for use in microprobe SLMS applications. First, because of a large ionization volume, the ion beam is of a low brightness and low ion current density (<5×10−7 A·cm−2). Second, the ionization efficiency of the apparatus depends on the equilibrium vapor pressure of the fullerene and activator molecules (AlF3+KF). Third, the final ion beam current is difficult to control and adjust over a wide range because the ion current continues so long as activator molecules 28 exist in external cell 24. Fourth, because external and internal cells 22 and 24 arc heated simultaneously using the same oven, it is impossible to efficiently achieve a combined optimal level of fullerene vapor pressure, activator vapor pressure and surface temperature of external cell 24. Fifth, the apparatus of FIG. 1 is inherently inefficient in using the fullerene powder due to intensive effusion of neutral fullerene molecules through the wide exit orifice 32 and also due to the destruction of a portion of the fullerene molecules by a catalytic reaction by interaction of the fullerenes with the hot nickel surface of external cell 24.
There is thus a widely recognized need for, and it would be highly advantageous to have an apparatus for generating neutral and anionic particulate beams, and a method for generating neutral and anionic particulate beams devoid of the above limitations. More particularly, it would be highly advantageous to have an apparatus and method for generating anionic and neutral fullerene beams.